1. Field of the Invention
The present invention relates to sensing data in memory devices, and more particularly to preventing disturbance of sensing operations in memory devices due to noise.
2. Description of Related Art
There are a variety of types of non-volatile memory based on charge storage memory cells, including memory cells that store charge between the channel and gate of a field effect transistor. The amount of charge stored affects the threshold voltage of the transistor, which can be sensed to indicate data.
One type of charge storage memory cell is known as a floating gate memory cell. In a floating gate memory cell, charge is stored on an electrically conductive layer between the channel and gate of the transistor. The threshold voltage is changed by adding or removing charge on the electrically conductive layer by applying appropriate voltages to the memory cell. Another type of memory cell is referred to as a charge trapping memory cell, which uses a dielectric charge trapping layer in place of the floating gate.
In a read or sense operation of a data value stored in a memory cell, appropriate voltages are applied to induce a current to flow from the drain terminal to the source terminal of the memory cell. The current is dependent upon the threshold voltage of the memory cell, and thus is indicative of the data value stored therein.
Determining the data value stored in the memory cell can be carried out using a sense amplifier which senses the current through the memory cell, and compares the sensed current to a suitable reference or references. FIG. 1 illustrates a prior art implementation of a sense amplifier 170 used for sensing a data value stored in a selected memory cell 110.
Memory cell 110 is representative of memory cells in a memory array which may include millions or billions of memory cells. Word line 120 is coupled to the gate terminal of the memory cell 110. Bit lines 130, 132 are coupled to the source and drain terminals 112, 114 of the memory cell 110. Column select transistor 140 is responsive to an SEL signal to selectively couple the bit line 130 to a data line 150 connected to a sensing input 172 of sense amplifier 170.
In a sense operation of the memory cell 110, appropriate voltages are applied to word line 120 and bit line 132 to induce a read current ICELL from the drain terminal 114 to the source terminal 112 and onto the bit line 130. The read current ICELL is provided to the data line 150 via the column select transistor 140. The read current ICELL charges an equivalent load capacitor CLOAD1 at the sensing input 172 of the sense amplifier 170, causing the voltage on the sensing input 172 to change by an amount proportional to the read current ICELL over the duration of the sense operation. The magnitude of the read current ICELL depends on the threshold voltage of the memory cell 110, with a lower threshold voltage resulting in a higher current. Thus, the voltage at the sensing input 172 will change more rapidly if the memory cell 110 is in a lower threshold state than if the memory cell 110 is in a higher threshold state.
A reference is used to provide a reference current IREF to a reference input 174 of the sense amplifier 170 during the sense operation. In this example, the reference current IREF is provided using a reference cell 210.
A word line 220 is coupled to the gate terminal of the reference cell 210. Bit lines 230, 232 are coupled to the source and drain terminals 212, 214 of the reference cell 210. Column select transistor 142 selectively couples the bit line 230 to a reference line 160 connected to the reference input 174 of the sense amplifier.
Appropriate voltages are applied to the word line 220 and the bit line 232 to induce a reference current IREF from the drain terminal 214 to the source terminal 212 and onto the bit line 230. The reference current IREF is provided to the reference line 160 via column select transistor 142. The reference current IREF charges an equivalent load capacitor CLOAD2 at the reference input 174 of the sense amplifier 170, converting the reference current IREF to a reference voltage on the reference input 174.
The sense amplifier 170 is activated by a sense enable signal SEN during the sense operation. The sense amplifier 170 is responsive to the difference between the voltage on the sensing input 172 and the voltage on the reference input 174 to generate an output signal 176 indicating the data value stored in the selected memory cell 110.
FIG. 2 is a simplified illustration of the change in voltage on the sensing input 172 and the reference input 174 during the sensing operation. Curve 250 illustrates the change in voltage at the sensing input 172 if the selected memory cell 110 is in the low threshold state. Curve 260 illustrates the change in voltage at the sensing input 172 if the selected memory cell 110 is in the high threshold state. The difference between the curves 250 and 260 at time T1 is a sensing margin used to distinguish the low threshold state from the high threshold state. In order to reliably distinguish between the high and low threshold states, it is important to maintain a relatively large sensing margin. In multiple-bit-per-cell embodiments, there are more than two threshold states, with sensing margins between them.
Curve 270 illustrates the change in voltage on the reference input 174 during the sensing operation. In this example, at time T1 curve 270 has a voltage between the low threshold state curve 250 and the high threshold state curve 260. This can be achieved, for example, by setting the threshold voltage of the reference cell 210 between the lower and higher threshold states of the memory cell 110, so that the reference current IREF has a magnitude between the read current ICELL of the memory cell 110 in the high threshold state and the low threshold state. As another example, this can be achieved by applying different voltages to the word lines 120 and 220, and/or applying different voltages to the bit lines 132 and 232.
The sense amplifier 170 generates an output signal 176 having a value dependent upon whether the voltage on the sensing input 172 is above or below the voltage on the reference input 174 at time T1, thereby indicating the data value stored in the memory cell 110.
An issue which arises during the sensing operation is the susceptibility of the sense amplifier 170 to noise. Specifically, noise occurring during the sensing operation can affect the difference between the voltages on the sensing input 172 and the reference input 174, which increases complexity of the sense amplifier 170 or the time needed for sensing.
In the implementation illustrated in FIG. 1, memory cell 110 is isolated from the reference cell 210, and the read current ICELL and the reference current IREF are not dependent upon one another. Consequently, the memory cell 110 can be exposed to noise different from that of the reference cell 210, which can cause disproportionate changes in the read current ICELL and the reference current IREF. This results in a wide variation in the voltage differential between the sensing input 172 and the reference input 174, which inhibits the sense amplifier 170 from accurately sensing the data value stored in the memory cell 110.
In the above described implementation, the sensing input 172 of the sense amplifier is coupled to the source terminal 112 of the memory cell 110 (“source-side sensing”). As a result, the voltage on the source terminal 112 will also increase by an amount dependent upon the read current ICELL. The increase in voltage on the source terminal decreases the drain-to-source voltage and increases the body effect of the memory cell 110. This in turn reduces the read current ICELL provided by the memory cell 110.
The threshold voltages of memory cells in an array will vary because of variations in the operating environment, as well as in materials and manufacturing processes. These variations result in differences in read current among memory cells storing the same data value, including differences in the change in read current caused by an increase in the source voltage. Thus, having the source voltage increase by an amount dependent upon the read current results in a wide distribution of the voltage or current at the sensing input 172 of the sense amplifier 170, which increases the complexity of the sense circuitry or the time needed for sensing. Source-side sensing circuitry and methods for operating addressing issues caused by the variation in source voltage have been proposed in U.S. patent application Ser. No. 12/576,466.
It is therefore desirable to provide sense circuitry and methods for operating such circuitry having low susceptibility to noise and addressing the issues caused by the variation in source voltage.